In recent years, various techniques for crystallizing or improving the crystallinity of an amorphous or polycrystalline semiconductor film have been investigated. This technology is used in the manufacture of a variety of devices, such as image sensors and active-matrix liquid-crystal display (AMLCD) devices. In the latter, a regular array of thin-film transistors (TFT) is fabricated on an appropriate transparent substrate, and each transistor serves as a pixel controller.
Sequential lateral solidification (SLS) using an excimer laser is one method for fabricating high quality polycrystalline films having large and uniform grains. A large-grained polycrystalline film can exhibit enhanced switching characteristics because the number of grain boundaries in the direction of electron flow is reduced. SLS processing controls the location of grain boundaries. U.S. Pat. Nos. 6,322,625, 6,368,945, 6,555,449 and 6,573,531 issued to Dr. James Im, the entire disclosures of which are incorporated herein by reference, and which are assigned to the common assignee of the present application, describe such SLS systems and processes.
In an SLS process, an initially amorphous or poly crystalline film (for instance, a continuous wave (CW)-processed Si film, an as-deposited film, or solid-phase crystallized film) is irradiated by a narrow laser beamlet. The beamlet is formed by passing a laser beam through a patterned mask, which is projected onto the surface of the film. The beamlet melts the precursor film, and the melted film then recrystallizes to form one or more crystals. The crystals grow primarily inward from edges of the irradiated area towards the center. After an initial beamlet has crystallized a portion of the film, a second beamlet irradiates the film at a location less than the lateral growth length from the previous beamlet. In the newly irradiated film location, crystal grains grow laterally from the crystal seeds of the polycrystalline material formed in the previous step. As a result of this lateral growth, the crystals attain high quality along the direction of the advancing beamlet. The elongated crystal grains are generally perpendicular to the length of the narrow beam and are separated by grain boundaries that run approximately parallel to the long grain axes.
Although the resultant polycrystalline films have elongated grains with enhanced mobilities, the many iterative steps of irradiation and translation result in low throughput rates. There is a need to increase the throughput rates in the processing of semiconductor materials without sacrificing the quality of the processed materials.